I. Field of the Invention
The present invention relates to a method for densifying previously sintered parts of powdered metals, ceramics and the like.
II. Description of the Prior Art
In the liquid phase sintering of powdered metals, ceramics and the like, the powdered material comprising a powdered hard phase and powdered binder is first intermixed with a fugitive binder which holds the part in the desired shape after cold pressing. Usually this fugitive binder or "wax" consists of a paraffin, polyetheleneglycol or a metal containing hydrocarbon. The cold pressed part is conventionally known as a preform.
The preforms are then subjected to a presintering step in which the preforms are slowly heated thus vaporizing the fugitive binder and the vaporized binder is removed from the part by a wash gas, vacuum pumping or other means. Following the presintering step, the parts retain their shape despite the absence of the fugitive binder due to some solid-state sintering of the powdered binder.
The parts are then subjected to a sintering operation in which the parts are raised to their liquid phase temperature which not only densifies the parts but also further releases any residual contaminants contained within the parts. These contaminants are removed from the part during the sintering operation by vacuum pumping or by flowing a wash gas, such as hydrogen, across the parts. Following the sintering of the parts, the parts are sufficiently dense and hard for many applications.
These sintered parts comprise hard phase particles such as tungsten, held together by the binder, such as cobalt. Following the sintering operation, the part contains many voids surrounded by a mix of hard phase particles and binder and in which the hard phase particles are spaced from each other by a distance less than the width of the void size.
For applications requiring still further densification, greater strength of the sintered part or better internal integrity, these properties of the part can be improved by subjecting the part to hot isostatic pressing or "HIP" processing. During HIP processing, the parts are ressurized to about 5000 psi and then elevated to their liquid phase temperature, for a period of 60 to 90 minutes. At this temperature, the pressure increases to above 10,000 psi due to thermal expansion. The primary advantage of HIP processing is to eliminate virtually all porosity within the part as well as greatly minimizing larger randomly spaced holes, slits or fractures which may be present in the part provided that such holes, slits or fractures are not open to the surface.
During the HIP process, as the parts are heated above solidus, the binder, e.g. cobalt, becomes molten and the spaces between the hard phase particles form capillary passageways which are open to the voids in the part. In the absence of pressure applied to the part the capillary force created by these passageways would prevent any molten binder contained within the part from entering the voids of the part.
During the HIP process, however, extremely high pressures, e.g. 5000 psi, are applied to the parts at a temperature below liquidus and this pressure is sufficient to overcome this capillary force once the material is heated above liquidus. Consequently, after the parts are heated above liquidus the high pressure forces the molten binder into the voids against the capillary force and results in what is well known in the art as "binder laking". Typically, the capillary force is about 1600 psi.
An example of such "binder laking" is shown in prior art FIG. 15 (1500X magnification) in which a 15% cobalt carbide part was subjected to the HIP process. FIG. 16 (500X magnification) also shows a Carboloy General Electric MPD grade 268 after HIP processing. Large cobalt lakes are evident throughout the parts in both FIGS. 15 and 16. Although laking is preferable to porosity, it is less preferable than a more homogenous microstructure for the part.
A still further disadvantage of HIP Processing is that, due to the high temperatures and high pressures used during the HIP processing, the previously known HIP equipment is extremely massive in construction and expensive to produce and acquire. Furthermore, the long cycle time for the HIP processing limits the production volume of HIP equipment and greatly increases the per part cost of the parts which are HIP treated.